 1:  1 + 2  = 3 
 2:  2 + 3  = 5 
 3:  3 + 4  = 7 
 4:  5 + 6  = 11 
 5:  6 + 7  = 13 
 6:  8 + 9  = 17 
 7:  9 + 10 = 19 
 8: 11 + 12 = 23 
 9: 14 + 15 = 29 
10: 15 + 16 = 31 
11: 18 + 19 = 37 
12: 20 + 21 = 41 
13: 21 + 22 = 43 
14: 23 + 24 = 47 
15: 26 + 27 = 53 
16: 29 + 30 = 59 
17: 30 + 31 = 61 
18: 33 + 34 = 67 
19: 35 + 36 = 71 
20: 36 + 37 = 73